I hiked in to this place starting at 4 in the morning not knowing what to expect. The "fire" is ignited sulphuric gas, which emerges from cracks at temperatures up to 600 degrees Celsius (thank you Wikipedia). Ijen crater is a working sulfur mine. That's what the miner in the picture is lighting up with a head lamp. To my right, is a highly acidic lake where it's rumored people have fallen in and never been recovered. Crazy! Totally worth the 4 am hike if you find yourself on the far east coast of Java Indonesia.